This invention relates generally to spread spectrum communications systems. In particular the disclosed apparatus and method for carrier recovery in MPSK modulated systems are based on correlation techniques.
Communication systems exist in various forms. Generally speaking, a communication system is designed to transmit information-embedding signals from a source to a destination. A communication system usually consists of three components: a transmitter, a channel and a receiver. The function of the transmitter is to process (also known as modulate) the information data into such a form that it can be transmitted through a channel. The channel provides a physical medium for signal transmission between the transmitter and the receiver. Examples of a channel are coaxial cable, optical fiber, and the air. A practical channel usually distorts the signal by addition of noises and time delay when the signal passes through it. The function of the receiver is to receive the transmitted signal and process (also known as demodulate) it to obtain the original information data. Since the signal received from a practical channel is usually distorted and delayed, the receiver requires mechanisms such as equalization, synchronization and so on to assist the data demodulation.
One type of communication systems is the mobile communication system, in which the signal is transmitted in the form of electromagnetic waves and the transmission medium is the air. The mobile phone communication system is a typical example of this type of communication system.
One type of mobile communication systems is Code Division Multiple Access (CDMA) system, which uses spread spectrum techniques. In this type of system the frequency bandwidth of the transmitted signal is much larger than the information bandwidth. Spread spectrum systems perform better in the presence of narrow band noise interference and multiple user interference. Direct Sequence Code Division Multiple Access (DS-CDMA) is one of the CDMA types. In a DS-CDMA system the transmitter spreads the data stream using a given spreading code in the time domain. Orthogonal codes are often employed among different users to minimize multi-user interference. The received signal is correlated with the user spreading code so that only the desired user signal is enhanced while signals from other users are de-emphasized. In this way users can share the same time and frequency slot.
Various modulation schemes can be used in DS-CDMA systems. One of the commonly used modulations is Multiple Phase Shift Keying (MPSK), in which the serial data is grouped into log2M-bit symbols. The well known binary phase shift keying (BPSK) and quadrature phase shift keying are particular ones of MPSK, where M=2 and M=4, respectively. Each information data bit is modulated with a pseudo-noise code (PN code) of length N. The spreaded data symbols are up-converted and transmitted by a carrier of frequency fc.